1. Field of the Invention
The present invention relates to a photovoltaic power generating system, more specifically to a photovoltaic power generating system with a snow-melting function.
2. Related Background Art
In recent years, photovoltaic power generating systems for residences with solar cells mounted on a roof have become popular. In a snowy district, however, snow sticks to the surface of a solar panel, thereby disabling power generation while shown is present. Accordingly, a snow removal system which entails heating a solar panel module has been invented.
In Japanese Patent Application Laid-Open No. 8-250756, for example, a snow-melting system is disclosed which supplies electric power to a solar cell or an exothermic body for heating with the aid of an external power source.
In Japanese Patent Application Laid-Open No. 9-23019, a snow-melting system is disclosed which uses a bidirectional inverter to supply electric power to a solar cell for heating with a switch provided for the bypass of a reverse-current preventive diode.
Since the solar cell is utilized not only as a generating device but also as a heater, the cell-heating scheme is simply capable of snow removal without the need for further wiring for a heater; it further becomes easy to install if the bidirectional operativity is afforded. Thus, it is promising as a snow removal method for the photovoltaic power generating system.
In the schemes disclosed thus far, however, the following problems may occur.
&lt;1&gt; Since whole array must be warmed, the capacity of an external power source increases. Heating by the flow of the same current through a solar cell, e.g., of 3 kWp in the forward direction as the optimum current, requires a power on the order of about 4.2 kW, and a capacity sufficient to supply this power is necessary for the power source. Especially for a system with the directional use of a grid-connected inverter, the capacity of the inverter increases, and the inverter of large capacity is required for the supply of a power for snow-melting not used at ordinary times.
&lt;2&gt; When supplying power all at once to the whole array, there is a possibility of current concentration in a solar cell circuit due to a decrease in resistance by unbalanced heating of a solar cell. In this application, the solar cell basically works as parallel connected diodes and requires a countermeasure for unbalanced heat-up. The occurrence of current concentration might lead to a critical accident such as damage of solar cells due to overheating.
&lt;3&gt; For these reasons, it is not possible to run the snow-melting mode if even one defective string is present.